1. Field of the Invention
The present invention relates to a light-emitting semiconductor device and especially to a light-emitting diode and an electroluminescence device and methods for making them.
2. Description of Related Art
According to the conventional light-emitting diodes made using ZnS, blue light emission has been obtained from a thin film type MIS structure comprising a semiconductor substrate and a ZnS layer containing an impurity, a high resistivity layer and an electrode layer which are successively laminated on a substrate. Japanese Patent Kokai (Laid-Open) Nos. 61-164275 and 61-172382 relate to light-emitting semiconductor devices of this kind. In these conventional light-emitting semiconductor devices, a low resistivity n-ZnS layer containing Al is used as an impurity-containing ZnS layer and a ZnS layer free from impurities is used as a high resistivity layer.
Furthermore, Japanese Patent Kokai (Laid-Open) No. 61-240592 has proposed use of a ZnS layer containing a halogen donor and an acceptor selected from the group consisting of Au, Ag and Cu as a light emitting layer for thin film electroluminescence.
The light-emitting diode of the above-mentioned conventional techniques, has the problems of low emission efficiency and low stability of emission spectrum. The blue light emission of this light-emitting diode is due to the recombination center of Al donor and acceptor produced by crystal defects. That is, a trivalent Al ion forms a donor level of -150 meV in ZnS crystal and the vacancy of Zn forms a deep acceptor level (-700 meV). There has been the problem that it is very difficult to control the concentration of the spontaneously produced acceptor in high reproducibility. Furthermore, there has been a problem in the stability of the emission spectrum because two emission bands are produced by a self activated (SA) center and a donor.acceptor (DA) center between Al donor and V.sub.Zn acceptor. An additional problem is that emission efficiency is determined by low hole injection efficiency from high resistivity layer.
Among the above-mentioned conventional techniques, thin film electroluminescence has the problem that the stability of the formed DA center and emission efficiency are inferior since the acceptor is doped by thermal diffusion.